Theoretical characterizations of the mechanism for the dimerization of monosilicic acid in basic solution

The anionic mechanisms for the elementary dimerization reaction of monosilicic acid in basic aqueous solution have been characterized comprehensively using various ab initio methods. Many new insights into the silicate oligomerization reaction, which is fundamentally important in sol-gel chemistry, zeolite synthesis, and cement hydration, are presented in this work. Conformational dependence of the dimerization reaction is proposed in view of hundreds of conformations with various inter- and intramolecular hydrogen bonding patterns along the reaction routes. An alternative water cleavage route from the five-coordinated silicon intermediate is revealed. The detour involves a six-center cyclic transition state, which is more preferable energetically than the well-known four-center water removal step. By including explicit water molecules, the activation barrier of the four-center water cleavage path can be reduced considerably to be even lower than the first barrier of the Si-O bond formation. In contrast, the six-center detour is less affected by the additional water molecules due to the unfavorable geometric distortion. The new understanding of the dimerization mechanism could have considerable impact on the initial stages of silica nucleation.